People with hearing loss often have great difficulty understanding speech in noisy environments, even with assistance from a hearing aid. Perceptual studies suggest that a deficit in temporal processing of rapidly varying fine structure is responsible. However, neurophysiological studies in animals have generally not found a deficit in temporal processing. Hence, the basis for poor speech intelligibility remains unclear. The existing neurophysiological studies are limited, however, because they measured temporal processing under quiet conditions and primarily examined only one commonly occurring form of hearing loss (noise- induced). Furthermore, these studies did not examine the frequency tuning of temporal responses to broadband stimuli. The proposed neurophysiological study of auditory nerve fiber responses in chinchillas resolves these limitations by (1) quantifying temporal processing in background noise, where current hearing aids often fail, (2) examining two forms of hearing loss, and (3) quantifying the frequency tuning of temporal responses. Aim 1 uses acoustic overexposures to study noise-induced hearing loss (NIHL), while Aim 2 uses furosemide injections to study metabolic hearing loss (MHL; furosemide injections simulate this common form of age-related hearing loss with good results). Acoustic stimuli will include pure tones and amplitude modulated tones. Temporal processing will be quantified using classical vector strength and novel correlation indices computed from shuffled correlograms. Finally, Aim 3 will employ Wiener kernel analyses of temporal responses to white noise to study the frequency tuning of temporal processing in animals with NIHL an MHL. The results of the proposed experiments will be used to test three hypotheses. First, hearing loss is expected to result in a deficit in temporal processing in background noise. This hypothesis is based on models of signal processing, which predict a stronger effect of background noise on the response to a signal following an increase in auditory filter bandwidth (i.e. more noise energy enters the filter during the processing task, thereby increasing masking). Second, based on the related studies of auditory perception, hearing loss should degrade processing of temporal fine structure more than the amplitude envelope. Finally, for a given degree of hearing loss, NIHL is expected to result in a greater deficit in temporal processing in noise than MHL. This hypothesis is based on previous observations of tuning curves with hypersensitive tails in studies of NIHL, but not MHL. The hypersensitive tails should allow more noise energy to enter during signal processing, thereby degrading temporal coding to a greater degree. The proposed research will advance our knowledge of the deficits in auditory processing associated with two common forms of hearing loss. The specific deficits identified by this and other studies can be used to guide the development of new technologies (e.g. hearing-aid and cochlear-implant designs) aimed at restoring speech perception under real-world listening conditions in people with hearing loss.